Centrifugal clutches are well known in the art. In essence, these devices, sometimes known as frictional contact axial clutches, utilize mating frictional members to transfer torque from an input housing to an output shaft. This is accomplished by harnessing the effects of centrifugal force upon radially translatable elements to generate axial movement and ultimately axial thrust. This axial thrust is applied upon an input housing frictional member which, by interacting with an output shaft frictional member, effectively transmits the input housing torque to the output, or driven shaft.
In one such type of axial clutch, the radially translatable elements are simple weights which surround the output shaft. Typically these weights are radially spring biased away from the input housing and against the output shaft. During operation, as the angular velocity of the input housing increases, likewise the centrifugal force developed by each of these weights increases. When the centrifugal force associated with each weight exceeds the preload force developed in each of the weight springs, these weights begin to move radially outwardly. These weights are commonly designed with a slanted surface such that continued outward radial movement of the weights will result in corresponding axial movement of a pressure plate which has a coacting slanted surface abutting the slanted surface of the weights. As the weights translate radially outwardly, the pressure plate continues translating axially compressing a series of clutch rings which are fixedly attached within the input housing. These clutch rings have disposed therebetween the peripheral edges of clutch disks, the disks themselves being splined to the output, or driven shaft. When the axial force applied by the pressure plate develops a sufficiently large frictional force between the clutch rings and the clutch disks so as to preclude slippage therebetween, the clutch becomes engaged and the output shaft commences rotation. The input housing angular velocity at which this engagement occurs is often referred to as the "engagement speed." One such clutch which provides for effecting fine tuning adjustments to account for the change in engagement speed due to frictional war of the clutch rings and disks to maintain a constant engagement speed, and for selectively adjusting the clutch engagement speed, is shown in the inventor's own U.S. Pat. No. 5,070,984.
In another such type of clutch, known as a "ball clutch," weighted balls are substituted for the weights. The balls ride in grooves in the pressure plate which are ramped such that radial movement of the balls causes axial movement of the balls along the ramped grooves. The balls are retained in the grooves by a clutch plate which is spring biased against the balls by one or more axially oriented compression springs. Outward radial movement of the balls upon acceleration causes axial movement of the balls which in turn causes axial movement of the clutch plate against the stiffness of the spring. A clutch disk positioned between the clutch plates is gripped by the clutch plates and thereby has torque transmitted to it and consequently to the output.
One problem associated with such ball clutches is frictional induced hysteresis. Friction between the balls and the ramped grooves and the clutch plate causes the clutch to engage at one speed yet disengage at another speed. This is because the balls travel radially outwardly upon acceleration more readily than they travel radially inwardly upon deceleration due to the frictional forces developed between the mating surfaces. Thus the clutch will engage at a higher speed, but will disengage at a lower speed. The cause of such hysteresis is due at least in part to the relatively inefficient action of the spring force resisting the balls; the direction of the spring force is axial whereas the direction of ball movement is radial and axial, resulting in required spring stiffnesses of on the order of 500 lb/in for this traditional type of ball clutch for each of six springs.
Another drawback to such traditional ball clutches is that they are generally expensive to manufacture. This is because the geometry of the ramped pressure plate generally requires that it be milled rather than fabricated as a cheaper stamping.
It is therefore one objective of the present invention to provide a ball clutch which is more efficient and which exhibits less hysteresis than traditional ball clutches.
It is another objective of the present invention to provide a ball clutch which is less expensive to manufacture than traditional ball clutches which include milled pressure plates.
It is yet another objective of the present invention to provide an axial clutch with the foregoing features the engagement speed of which is easily selected and controlled.
It is still another objective of the present invention to provide an axial clutch with the foregoing features for which the replacement of the consumable parts of the clutch, i.e. clutch drive plates and disks, is simply and quickly accomplished.